Power cycles for WTP (Waste heat to Power) are well described, such as ORC, Kalina, Trilateral Flash etc.
Such power cycles are designed to recover waste heat from the heat source and to transform said energy into useful mechanical energy that can be used for instance for driving a generator for generating electrical power.
The use of an ORC (Organic Rankine Cycle) is in particular known to recover waste energy of heat sources with relatively low temperature like the heat of compressed gas produced by a compressor installation, or comprised in exhaust gasses, flue gasses, steam, hot water or the like.
Such known ORC's comprise a closed loop circuit containing a two-phase working fluid, the circuit further comprising a liquid pump for circulating the fluid in the circuit consecutively through an evaporator which is in thermal contact with the heat source to evaporate the working fluid; through an expander like a turbine for transforming the thermal energy transmitted to the gaseous working fluid produced in the evaporator into useful mechanical energy; and finally through a condenser which is in thermal contact with a cooling medium like water or ambient air in order to transform the gaseous working fluid into liquid that can be returned to the evaporator for the next working cycle of the working fluid.
In installations producing hot gasses, the ORC is used for cooling said hot gasses by bringing these hot gasses in contact with the evaporator of the ORC and at the same time to use the ORC for transforming the heat recovered in the evaporator into useful energy in the expander.
A disadvantage of the existing ORC's is that the size of the evaporator has to be relatively large in order to have a sufficient heat transfer contact between the working fluid in the evaporator and the heat source, especially with a low temperature heat source of for example 90° C. or even 60° C., the contact surface between the liquid fraction of the working fluid to be evaporated in the evaporator being only a small fraction of the total contact surface of the evaporator since the evaporator only contains liquid at the bottom and vapors of the working fluid on top of it.
Another disadvantage is that in case of a failure of the liquid pump or expander, the circulation of the working fluid in the ORC comes to a halt automatically, since the evaporator needs to be located above the expander in order to provide a gravitational flow of the liquid fraction of the fluid from the evaporator to the expander, especially when a two-phase fluid to the inlet of the expander is preferred.
When the working fluid in the ORC stops circulating, the cooling function of the ORC to cool the hot gasses is lost, leading to potentially dangerous situations whereby the downstream installations or downstream users making use of the uncooled hot gasses could be damaged due to overheating.
The unpublished Belgian patent application 2014/0654 of the same applicant provides for a solution in case of failure of the liquid pump of the ORC by introducing auxiliary coolers which are not part of the ORC system and which therefore can secure cooling the compressed gasses in case of failure of the ORC system.
A disadvantage is that auxiliary coolers have to be provided.